In the oil and gas exploration and extraction industries it is often desired to lock or set tools within a bore to allow, for example, the bore to be sealed or to provide a mounting or carrier for other tools to be located within the bore. Conventionally, such tools are located in the bore by means of keys or dogs which are expanded to engage with an annular recess in a restriction in the bore, known as a locating nipple. In order to permit manipulation of the tool to expand the keys a shoulder, known as a no-go, is provided adjacent the recess or nipple profile and provides a stop for the tool and a point against which force which can be applied for manipulation and setting of the tool. Such no-gos provide a relatively straightforward and simple means for location of such tools, it merely being necessary to provide the tool with, for example, a collar sized to engage a particular no-go as the tool is run into a bore on the end of a tool string. However, each no-go reduces the diameter of the bore and thus the provision of a number of no-gos in a bore significantly reduces the minimum internal diameter of the bore. This can be detrimental to the wellbore for various reasons, not least that the flow area of the bore which oil or gas can be produced through is reduced.
In recognition of this problem there have been various proposals for "selective" locks, which may be located and set in a bore without requiring the provision of no-gos in the bore. However, in order to permit locking keys to be set, these locks are provided with arrangements for engaging the bore wall prior to the keys being set. This has resulted in relatively complex systems and the additional point of engagement between the lock and the bore wall can lead to difficulties when attempting to remove such locks. Locks of this form currently available include the SUR-SET selective lock supplied by Baker Packers, the Otis X and Otis R lock mandrels, and the Camco Series DB landing locks.
It is among the objects of the present invention to obviate or mitigate these disadvantages.